Influence of temperature and orientation on the mechanical properties of amorphous regions on the example of polyethylene

Abstract

In this work, the influence of temperature and molecular orientation on the elastic modulus of the amorphous regions (E_a) of semicrystalline polymers was determined using high-density polyethylene (HDPE) as an example. The decrease in E_a with increasing temperature was observed due to the gradual increase in the mobility of polymer chains, stimulated by the α relaxation processes within the crystalline regions. Thus, with the temperature increase, a gradual minimization of the “stiffening” influence of the crystals on the interlamellar layers was observed. Then, the influence of both the orientation and the undisturbed length of crystallites, as well as the orientation of the amorphous component, on the E_a value was demonstrated using materials deformed in a channel die with different compression ratios. At low compression ratios, a significant decrease in E_a was observed, stimulated by the fragmentation of lamellar crystals in the absence of measurable molecular orientation. At higher compression ratios, although the processes of crystal fragmentation remain active, a gradual increase in the orientation of chains in both the crystalline and amorphous components along the E_a measurement direction stimulated a significant increase in the E_a value. Consequently, E_a for the material with the highest compression ratio was six times higher compared to the reference material. The observed changes in E_a were also correlated with changes in the macroscopic Young's modulus.